Heat recovery from a high pressure stream

ABSTRACT

A process for recovering heat from a high pressure stream during hydroprocessing, where one embodiment of the process includes serially introducing a high pressure stream from a hot separator into a first steam generator and a second steam generator; using the first steam generator to generate a medium pressure stream of steam, and then using the medium pressure stream as stripping steam. The process also includes using the second steam generator to generate a low pressure stream of steam, and then using the low pressure stream as stripping steam.

The present invention relates generally to hydroprocessing units, andmore particularly to a process for recovering heat from a high pressurestream. In one embodiment, the high pressure stream providing the heatis a vapor stream from a hot separator, which is used to generate both amedium steam and a low pressure steam that can each be used in furtherprocessing, such as being used as stripping steam within components suchas in a stripper, a product fractionator, and/or a diesel side stripper

BACKGROUND OF THE INVENTION

Energy optimization for hydroprocessing units, such as hydrocrackingunits, has become very important, and there is a drive towards minimumutilities and maximum heat recovery. The present inventors have realizedthat one way to achieve this is via steam generation using the hotseparator vapor. However, the present inventors also realize that sincethe hot side is reactor effluent that is at a very high pressure, safetyis a big concern. Hence, steam generation with the required intrinsicsafety becomes important. The scheme developed by the present inventors,an example of which is described below, achieves this requirement.

BRIEF SUMMARY OF THE INVENTION

Briefly, in certain embodiments, the present process is a process forrecovering heat from a high pressure stream during hydroprocessing,where one embodiment of the process includes serially introducing a highpressure stream from a hot separator into a first steam generator and asecond steam generator; using the first steam generator to generate amedium pressure stream of steam, and then using the medium pressurestream as stripping steam. The process also includes using the secondsteam generator to generate a low pressure stream of steam, and thenusing the low pressure stream as stripping steam.

Also, in certain embodiments, the present process is for recovering heatfrom high pressure steam during hydroprocessing includes the steps ofusing a hot separator to create a high pressure vapor stream, and thenextracting heat from the high pressure vapor stream to generate bothmedium pressure steam and low pressure steam. In certain embodiments,the medium pressure steam is routed to a stripper, where the mediumpressure steam is used as stripping steam, and the low pressure steam isrouted to at least one of a product fractionator and a diesel sidestripper, where the low pressure steam is used as stripping steam.

Finally, certain embodiments of the present process for recovering heatfrom high pressure steam during hydroprocessing involve routing a highpressure stream to a first process vessel and routing a first feed waterstream to the first process vessel. The process continues by extractingheat from the high pressure stream within the first process vessel tocreate a medium pressure stream of steam from the first feed waterstream. The process also involves routing the high pressure stream fromthe first process vessel to a second process vessel and routing a secondfeed water stream to the second process vessel. Finally, the processinvolves extracting heat from the high pressure stream within the secondprocess vessel to create a low pressure stream of steam from the secondfeed water stream.

BRIEF DESCRIPTION OF THE DRAWING

A preferred embodiment of the present invention is described herein withreference to the drawing wherein:

FIG. 1 is an example of an embodiment of the present process forrecovering heat from a high pressure stream within a hydrocracking unit.

DETAILED DESCRIPTION OF THE INVENTION

Briefly, in certain embodiments of the present process, which can beused in a hydroprocessing unit (such as a hydrocracking unit), twodifferent pressure levels of steam are generated using hot separatorvapor, where one of the steam streams is intended for use as strippingsteam for a stripper (medium pressure steam) MLP and the other of thesteam streams is intended for use as stripping steam for a productfractionator and a diesel side stripper (Low pressure steam). Each steamgenerator will produce exactly the amount needed for stripping at therequired level. Additional high pressure steam from the header will beused to makeup the medium pressure steam requirement, and additionalmedium pressure steam from the header will be used to makeup the lowpressure steam requirement when needed, such as during start-up and inother cases when steam generation is insufficient for the processrequirements. If the resulting steam generation is more than thatrequired, a pressure controller will close the streams of makeup steamfrom the header.

However, this closure causes the pressure in the steam generator(s) toincrease, which increases the temperature of the steam being generated.The result is that the temperature difference between the hot side fluidand the water from which the steam is generated will decrease and lesssteam will be generated. This will tend to self regulate the steamgeneration. Normally, the steam generators operate at a pressure lowerthan the respective header that supplies steam during startup. Toprevent contamination of the respective supply header, if the steampressure increases over a certain level, a high pressure switch closesan isolation valve to prevent the flow of steam back to the header. Thistype of valve closure prevents contamination of the steam header ifthere is a tube rupture or leak from the high pressure side. Such a tubeleak or rupture could otherwise cause hydrogen sulfide and othernon-condensibles to enter the steam header, thereby contaminating it.

In the case of a tube rupture, pressure in the steam drum will increaseand a high pressure switch will be activated, which will then closeshut-off valves in the boiler feed water (BFW) line and the blowdownline, thereby capturing the fluid from the rupture tube within thegenerator itself and minimizing contamination of the boiler feedwaterheader. The set pressure at which the switch gets activated is the BFWpump shut-in pressure. In certain embodiments, the steam generatordesign pressure is preferably set to be 10/13th of the tube side (highpressure) design pressure. After the steam generator has been cut-offfrom the BFW line, the steam outlet line, the makeup line and theblowdown line, the steam generator is isolated and a pressure safetyvalve

(PSV) on the generator will open if the pressure reaches the PSV setpressure. The line from the PSV is routed to the relief header, ratherthan to the atmosphere, since hydrocarbons and hydrogen sulfide will bepresent in the vapors to be relieved if there is a tube rupture. Sincethe PSV line is routed to the relief header, there is a chance ofleakage of steam to the flare header during normal operation, and hencea rupture disc is also provided upstream of the PSV to eliminateleakage. Otherwise, if steam leakage were to occur during coldertemperatures, a blockage of the relief header due to ice buildup couldresult.

In an example of one embodiment, as shown in FIG. 1, the present processis shown as being incorporated into a hydrocracking unit. Ashydrocracking units are known to those of ordinary skill in the art,only those process flows and components related to the present processare shown and described, as it should be clear to one of ordinary skillin the art how the present process can be incorporated into ahydrocracking unit. Also, it should be noted that the present process isnot limited to hydrocracking units, but can instead be provided intoother types of hydroprocessing units, as well as into processing unitsof other types in which heat recovery from a high pressure stream isdesired.

Turning again to FIG. 1, this figure shows an embodiment in which a highpressure vapor stream 10 is fed from a hot separator 12 to a firstprocess vessel used as a first steam generator, such as first cooler 22.Other embodiments are also contemplated, such as embodiments includingshell and tube exchangers arranged in parallel which exchange heat withboiler feedwater flowing by natural circulation from a vessel mountedabove the shell and tube exchangers. This vessel acts as a disengagingspace to separate the steam generated from the circulating boilerfeedwater. In this way, multiple services generating steam at the samepressure share a common separation vessel.

In an example of the FIG. 1 embodiment, the pressure of the stream 10could be within the range of 500 psig (34.5 barg) to 2800 psig (154barg), and the temperature could be within the range of 400° F. (200°C.) to 700° F. (370° C.). Of course, in other configurations, the streamwould be at a different pressure and temperature.

Prior to reaching the first cooler 22, the stream 10 can be passedthrough other components, such as through one or a series of heatexchangers, in order to remove some of the heat for use in other partsof the process. In this example, the stream 10 first passes through aheat exchanger 14 (such as a shell and tube heat exchanger), which heatsone of the process streams, such as fresh feed; it then passes throughanother heat exchanger (such as another shell and tube heat exchanger)16, which heats another stream in the process, such as recycle gas; andfinally it passes through a heat exchanger 20, which heats anotherstream in the process, such as feed to the fractionation section. Ofcourse, other configurations are also contemplated, depending on thevarious temperature and pressure parameters and the other components ofthe processing unit.

After the stream 10 has passed through the heat exchangers 14, 16, and20, the resultant stream 29 is routed to the first cooler 22, asmentioned above, wherein it used to provide heat to generate steam fromthe boiler feed water that enters cooler 22 through boiler feed water(BFW) line 25. The liquid level within the first cooler 22 is monitoredby a liquid level controller (LIC) 17 that is associated with a flowindicator controller 19 and a valve 21, for regulating and controllingthe amount of boiler feed water routed to cooler 22 through the boilerfeed water (BFW) line 25.

The boiler feed water is turned into steam within the first cooler 22 byextracting heat from stream 29, resulting in a resultant stream 24 ofsaturated steam. The resultant stream 24 could, for example, be at apressure within the range of approximately 100 to approximately 400 psig(7 to 28 barg) in other embodiments. After the resultant stream 24leaves the first cooler 22, it is fed to a superheater 26. As the steampasses through superheater 26, the saturated steam is superheated andleaves as stream 27, which can ultimately be fed to a stripper (notshown) through line 31, after passing through flow control valve 30.Valve 30 is controlled by an associated flow indicator controller thatregulates and monitors the flow of the stream to the steam strippercolumn.

However, prior to going through line 31 to be used as stripping steam inthe stripper, the superheated steam is mixed with a stream 33 of highpressure steam from the header. In order to arrive at the desiredpressure for the stripper (which in this case is the medium pressuresteam), this embodiment uses a control valve 35 associated with apressure indicator controller (PIC) 37, as well as an additional controlvalve 39 associated with an additional PIC 41. In particular, the PIC 37monitors the pressure of stream 27 at a point after this stream passesthrough superheater 26, but before being combined with another stream,and if the pressure of stream 27 needs to be increased (or decreased) inorder to arrive at the desired pressure for entering the stripperthrough line 31, PIC 37 opens (or partially or fully closes) valve 35 sothat more (or less) high pressure stream 33 is mixed with stream 27.

In this embodiment, the medium pressure stream of line 31 could be anypreselected pressure value between approximately 100 psig (7 barg) and400 psig (28 barg).

If PIC 41 determines that the pressure in line 43 is above apredetermined value (such as, for example, a predetermined value between140 psig (10 barg) and 300 psig (21 barg)), a high pressure switchcloses the isolation valve 39 to prevent the flow of steam to the highpressure header. Such a configuration prevents contamination of thesteam header if there is a tube leak or rupture on the high pressureside because without the closure of the isolation valve 39, hydrogensulfide and other non-condensables could enter the steam header during atube leak or rupture, thereby contaminating the header.

A pressure alarm system 100, which in this case is a pressure alarm(high/high), or PAHH, is associated with the first cooler 22 (firststeam generator). As known in the art, such pressure alarm systems, aswell as the other controls and controllers mentioned herein, arecommonly associated with a computer processor. This first pressure alarmsystem 100 includes a pressure indicator (PI) 102 that monitors thepressure of stream 24 at a location between first cooler 22 andsuperheater 26, as well as including shut-off valves 104, 106 and 108.If there is a tube rupture in first cooler 22, pressure within the firstcooler 22 will increase, and such an increase will be detected by thepressure indicator 102. Once the pressure reaches a predetermined level(such as, for example, a predetermined value between 140 psig (10 barg)and 300 psig (21 barg)), the controller activates a high pressure switchthat closes the following shut-off valves: (a) the shut-off valve 104(associated with stream 27), (b) the shut-off valve 106 (associated withblow down line 23), and (c) the shut-off valve 108 (associated with theboiler feed water line 25). Thus, with these valve closings, the fluidfrom the ruptured tube is safely captured within the first steamgenerator itself.

Further, once the shut-off valves 104, 106 and 108 have been closed andthe first steam generator (including the first cooler 22 in thisembodiment) is isolated, a pressure safety valve (PSV) 110 is configuredand arranged to open if the pressure reaches the PSV set pressure. Thestream from the pressure safety valve 110, when opened, is routedthrough stream 112 to a relief header (not shown) because, in thisembodiment, hydrocarbon and hydrogen sulfide will also be releasedduring a tube rupture. However, since in this embodiment the stream 112is routed to the relief header (not shown), there is a chance of leakageof steam to the relief header, and accordingly this embodiment alsopreferably includes a rupture disc 114, or other equivalent device, inseries with the PSV 110 to eliminate such steam leakage. If such steamleakage were to occur during colder temperatures, a blockage of theflare header could result.

The steam generator design pressure, which is the same as the PSV setpressure of this first steam generator (including first cooler 22) ispreferably set to be 10/13^(th) of the tube side design pressure in thisembodiment.

The present embodiment of FIG. 1 also includes a second steam generator,such as second cooler 32. Exit stream 60 from the first cooler 22 isused as the heat source for creating steam within the second cooler 32.Of course, the temperature of the stream 60 exiting the first cooler 22will be lower than that of stream 29 entering the first cooler 22because some of the heat has been extracted to create the steam of thestream 24 from the boiler feed water.

After stream 60 passes through the second cooler 32 and is used togenerate steam within the second cooler, an exit stream 62 from thesecond cooler 32 can be passed through one or more heat exchangers, orother components, before a resultant stream 64 is routed to a productcondenser for further processing, which processing is known to those ofordinary skill in the art. In the FIG. 1 embodiment, stream 62 is firstrouted to an exchanger 66, which may be associated with a recycle gasstream, and then to a heat exchanger 68, which receives a feed from acold flash drum (not shown). Of course other configurations are alsocontemplated, depending on the various temperature and pressureparameters and the other components of the processing unit.

Finally, the line associated with the blowdown stream 23 from the firstcooler 22 includes a valve 76, in addition to the valve 106 of the firstpressure alarm system 100 discussed above. This valve 106 is used tocontrol the flow of the blowdown stream 23, which is then designated asstream 77 after passing through the valve 106, and stream 77 is routedto a blowdown drum (not shown). In this example, the refinery blowdownnetwork is designed for low pressure, so the blowdown drum will act as avessel with a PSV where a pressure break can be achieved.

The second steam generator (second cooler) 32 operates in a similarmanner to that of the first steam generator (first cooler) 22, and thuswill not be described in great detail, except to discuss any significantdifferences between the two steam generators (coolers). Additionally,components and flows associated with the second cooler 32 thatcorrespond to those of the first cooler 22 will be designated with likereference numerals, except those associated with the second cooler willinclude a single prime (′) or a double prime (″) designation.

One difference between the flows of the steam generated with the secondcooler 32 and those associated with the first cooler 22 is instead ofhaving the resultant medium pressure stream 31 being passed to astripper (as with the first steam generator with the first cooler 22),the resultant stream from the second steam generator (with the secondcooler 32) is routed in parallel through two streams, designated as lowpressure stream 31′ and low pressure stream 31″. In this embodiment, thestream 31′ is routed to a product fractionator (not shown) and thestream 31″ is routed to a diesel stripper (not shown). The steam ofstreams 31′ and 31″ is used as the stripping steam in the productfractionator and the diesel stripper, respectively. In this embodiment,the streams 31′ and 31″ are preferably configured to be at a specificpredetermined pressure that is between 15 and 20 psi, but otherpressures are also contemplated, depending on the intended use of thestreams.

An additional difference between the flows associated with the first andsecond steam generators relates to the supplemental steam being providedarrive at the desired pressure for the medium pressure stream 31(associated with the first steam generator, including first cooler 22)and the low pressure streams 31′ and 31″ (associated with the secondsteam generator, including second cooler 32). In particular, with regardto the medium pressure stream 31, this stream can be mixed with theappropriate amount of high pressure steam from the header through stream33 to arrive at the predetermined pressure via various valves andcontrols, as discussed above. A similar control process is followed forthe low pressure streams 31′ and 31″, except instead of receivingsupplemental high pressure steam from the header through stream 33, asneeded, the low pressures streams 31′ and 31″ in this embodiment receivesupplemental medium pressure steam from the header, as needed.

Other than the differences noted above, the components associated withthe second steam generator (including the second cooler 32), such as thesecond pressure alarm system 101′, the second superheater 26′, etc.,operate in essentially the same manner as the corresponding componentsof the first steam generator (including the first cooler 22). According,such components need not be discussed further.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. It should also beappreciated that the exemplary embodiment or exemplary embodiments areonly examples, and are not intended to limit the scope, applicability,or configuration of the invention in any way. Rather, the foregoingdetailed description will provide those skilled in the art with aconvenient road map for implementing an exemplary embodiment of theinvention. It is understood that various changes may be made in thefunction and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

What is claimed is:
 1. A process for recovering heat from a highpressure stream during hydroprocessing, the process comprising: seriallyintroducing a high pressure stream from a hot separator into a firststeam generator and a second steam generator; using the first steamgenerator to generate a medium pressure stream of steam, and then usingthe medium pressure stream as stripping steam; and using the secondsteam generator to generate a low pressure stream of steam, and thenusing the low pressure stream as stripping steam.
 2. The processaccording to claim 1, further comprising: routing the medium pressurestream to a stripper; and routing the low pressure stream to at leastone of a product fractionator and a diesel side stripper.
 3. The processaccording to claim 2, wherein a portion of the low pressure stream isrouted to the product fractionator and a portion of the low pressurestream is routed to the diesel side stripper.
 4. A process forrecovering heat from a high pressure stream during hydroprocessing, theprocess comprising: using a hot separator to create a high pressurevapor stream; generating both a medium pressure stream of steam and alow pressure stream of steam from the high pressure vapor stream byextracting heat from the high pressure vapor stream; routing the mediumpressure stream to a stripper, where the medium pressure stream is usedas stripping steam; and routing the low pressure stream to at least oneof a product fractionator and a diesel side stripper, where the lowpressure stream is used as stripping steam.
 5. The process according toclaim 4, wherein a portion of the low pressure stream is routed to theproduct fractionator and a portion of the low pressure stream is routedto the diesel side stripper.
 6. The process according to claim 4,further comprising: routing the high pressure vapor stream from the hotseparator to a first steam generator, and extracting heat from the highpressure stream to form the medium pressure steam stream; routing thehigh pressure vapor stream, from which heat has been extracted in thefirst steam generator, to a second steam generator; and using the secondsteam generator to extract additional heat from the high pressurestream, thereby forming the low pressure steam stream.
 7. The processaccording to claim 6, further comprising: providing at least one firstheat exchanger between the hot separator and the first steam generator,and extracting heat from the high pressure stream with the at least onefirst heat exchanger; and providing at least one second heat exchangerafter the second steam generator and extracting additional heat from thehigh pressure stream with the at least one second heat exchanger.
 8. Theprocess according to claim 6, further comprising: providing a mediumpressure steam superheater between the first steam generator and thestripper column, and routing the medium pressure stream from the firststeam generator through the medium pressure steam superheater; andproviding a low pressure steam superheater after the second steamgenerator, and routing the low pressure stream from the second steamgenerator through the low pressure steam superheater.
 9. The processaccording to claim 10, further comprising: providing a pressure alarmsystem that is configured and arranged to activate a valve associatedwith a boiler feed water line at a predetermined high pressure value,wherein the boiler feed water line provides boiler feed water to boththe first steam generator and the second steam generator; and activatingthe valve associated with the boiler feed water line, when the pressurealarm system detects that the pressure has reached the predeterminedhigh pressure value, to stop flow of the boiler feed water to both thefirst steam generator and the second steam generator.
 10. The processaccording to claim 4, further comprising: controlling, using a computerprocessor configured and arranged to control at least one valve, thepressure of the medium pressure stream entering the stripper to be at afirst predetermined pressure of approximately 130 psig (9 barg); andcontrolling, using the computer processor configured and arranged tocontrol at least one valve, the pressure of the low pressure streamentering at least one of the product fractionator and the dieselstripper to be at a second predetermined pressure of betweenapproximately 15 psig (1 barg) and approximately 20 psig 1.4 barg). 11.The process according to claim 10, wherein: during the controlling ofthe pressure of the medium pressure stream, a supplemental high pressurestream of high pressure steam is controlled to be combined with themedium pressure stream to arrive at the first predetermined pressure;and during the controlling of the pressure of the low pressure stream, asupplemental medium pressure stream of medium pressure steam iscontrolled to be combined with the low pressure stream to arrive at thesecond predetermined pressure.
 12. The process according to claim 4,further comprising: combining the medium pressure stream with highpressure header steam to provide the stripper with a stream at apredetermined pressure; combining the low pressure stream with mediumpressure header steam to provide the diesel side stripper with a streamat a predetermined pressure; and combining the low pressure stream withmedium pressure header steam to provide the product fractionator with astream at a predetermined pressure.
 13. The process according to claim6, wherein: the first steam generator comprises a first cooler; and thesecond steam generator comprises a second cooler.
 14. A process forrecovering heat from a high pressure stream during hydroprocessing, theprocess comprising: routing a high pressure stream to a first processvessel; routing a first feed water stream to the first process vessel;extracting heat from the high pressure stream within the first processvessel to create a medium pressure stream of steam from the first feedwater stream, routing the high pressure stream from the first processvessel to a second process vessel; routing a second feed water stream tothe second process vessel; and extracting heat from the high pressurestream within the second process vessel to create a low pressure streamof steam from the second feed water stream.
 15. The process according toclaim 14, further comprising: providing a first pressure alarm systemthat is triggered by pressure above a first predetermined level, whereinsaid first pressure alarm system isolates the first process vessel bystopping flow of said first feed water stream into said first processvessel and by stopping flow of said medium pressure stream; andproviding a second pressure alarm system that is triggered by pressureabove a second predetermined level, wherein said second pressure alarmsystem isolates the second process vessel by stopping flow of saidsecond feed water stream into said second process vessel and by stoppingflow of said low pressure stream.
 16. The process according to claim 14,further comprising: routing the medium pressure stream to a firstsuperheater to create a first superheated stream of medium pressuresteam; using the first superheated stream as stripping steam; routingthe low pressure stream to a second superheater to create a secondsuperheated stream; and using the second superheated stream as strippingsteam.
 17. The process according to claim 16, further comprising: usingthe first pressure alarm system to stop flow of a first blowdown streamfrom the first process vessel; and using the second pressure alarmsystem to stop flow of a second blowdown stream from the second processvessel.
 18. The process according to claim 16, further comprising:providing a first pressure alarm system that is triggered by pressureabove a first predetermined level, wherein said first pressure alarmsystem isolates the first process vessel by stopping flow of said firstfeed water stream into said first process vessel and by stopping flow ofsaid medium pressure stream; determining a first pressure of said mediumpressure stream between the first process vessel and the firstsuperheater, and comparing the first pressure with the firstpredetermined pressure; activating the first pressure alarm system ifthe first pressure exceeds the first predetermined pressure; providing asecond pressure alarm system that is triggered by pressure above asecond predetermined level, wherein said second pressure alarm systemisolates the second process vessel by stopping flow of said second feedwater stream into said second process vessel and by stopping flow ofsaid low pressure stream; determining a second pressure of said lowpressure stream between the second process vessel and the secondsuperheater, and comparing the second pressure with the secondpredetermined pressure; and activating the second pressure alarm systemif the second pressure exceeds the second predetermined pressure. 19.The process according to claim 15, further comprising: providing a lineto rout a first effluent stream from the first processing vessel to arelief header; providing a first safety valve configured and arranged tobe normally closed, but to allow flow of said first effluent stream tothe relief header when activated; providing a line to rout a secondeffluent stream from the second processing vessel to the relief header;providing a second safety valve configured and arranged to be normallyclosed, but to allow flow of said second effluent stream to the reliefheader when activated.
 20. The process according to claim 19, wherein:said first safety valve is activated to allow flow of said firsteffluent stream if said first pressure alarm system has been activatedand if a first predetermined pressure safety valve pressure has beenreached; and said second safety valve is activated to allow flow of saidsecond effluent stream if said second pressure alarm system has beenactivated and if a second predetermined pressure safety valve pressurehas been reached.